Not applicable.
Not applicable.
The present invention relates generally to drill bits used in the oil and gas industry and more particularly, to drill bits having diamond-impregnated cutting surfaces. Still more particularly, the present invention relates to drag bits in which the diamond particles imbedded in the cutting surface have not suffered the deleterious thermal exposure that is normally associated with the manufacture of such bits.
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. When weight is applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
Different types of bits work more efficiently against different formation hardnesses. For example, bits containing inserts that are designed to shear the formation frequently drill formations that range from soft to medium hard. These inserts often have polycrystalline diamond compacts (PDC""s) as their cutting faces.
Roller cone bits are efficient and effective for drilling through formation materials that are of medium to hard hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action, in that the inserts of the rotating cones are impacted against the formation material. This action compresses the material beyond its compressive strength and allows the bit to cut through the formation.
For still harder materials, the mechanism for drilling changes from shearing to abrasion. For abrasive drilling, bits having fixed, abrasive elements are preferred. While bits having abrasive polycrystalline diamond cutting elements are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations such as sandstone. For these hard formations, cutting structures that comprise particulate diamond, or diamond grit, impregnated in a supporting matrix are effective. In the discussion that follows, components of this type are referred to as xe2x80x9cdiamond impregnated.xe2x80x9d
During abrasive drilling with a diamond-impregnated cutting structure, the diamond particles scour or abrade away concentric grooves while the rock formation adjacent the grooves is fractured and removed. As the matrix material around the diamond granules is worn away, the diamonds at the surface eventually fall out and other diamond particles are exposed.
To form a diamond-impregnated bit, the diamond, which is available in a wide variety of shapes and grades, is placed in predefined locations in a bit mold. Alternatively, composite components, or segments comprising diamond particles in a matrix material such as tungsten carbide/cobalt (WCxe2x80x94Co) can be placed in predefined locations in the mold. Once the diamond-containing components have been positioned in the mold, other components of the bit are positioned in the mold. Specifically, the steel shank of the bit is supported in its proper position in the mold cavity along with any other necessary formers, e.g. those used to form holes to receive fluid nozzles. The remainder of the cavity is filled with a charge of tungsten carbide powder. Finally, a binder, and more specifically an infiltrant, typically a nickel brass alloy, is placed on top of the charge of powder. The mold is then heated sufficiently to melt the infiltrant and held at an elevated temperature for a sufficient period to allow it to flow into and bind the powder matrix or matrix and segments. For example, the bit body may be held at an elevated temperature ( greater than 1800xc2x0 F.) for on the order of 0.75 to 2.5 hours, depending on the size of the bit body, during the infiltration process. By this process, a monolithic bit body that incorporates the desired components is formed. It has been found, however, that the life of both natural and synthetic diamond is shortened by the lifetime thermal exposure experienced in the furnace during the infiltration process. Hence it is desired to provide a technique for manufacturing bits that include imbedded diamonds than have not suffered the thermal exposure that is normally associated with the manufacture of such bits.
Another type of bit is disclosed in U.S. Pat. Nos. 4,823,892, 4,889,017, 4,991,670 and 4,718,505, in which diamond-impregnated abrasion elements are positioned behind the cutting elements in a conventional tungsten carbide (WC) matrix bit body. The abrasion elements are not the primary cutting structures during normal bit use. Hence, it is further desired to provide a bit that includes diamond particles in its primary or leading cutting structures without subjecting the diamond particles to undue thermal stress or thermal exposure.
The present invention provides a bit with cutting structures that include diamond particles, in which a portion of the diamond particles have not been subjected to undue amounts of thermal stress or thermal exposure. Specifically, the present invention comprises a bit that includes diamond-impregnated inserts as the cutting structures on at least one blade of the bit. The diamond-impregnated inserts are manufactured separately from the bit body. Once formed, the diamond-impregnated inserts are affixed to the bit body by brazing or other means of attachment. The total thermal exposure of the diamond particles during manufacture in accordance with the present invention is significantly lower than the total manufacturing-related thermal exposure in previously known diamond-impregnated cutting structures. Thus, the operating life of the cutting structures, and therefore the life of the bit itself, is increased.